Acute viral hepatitis is a disease which may result in chronic liver damage. It is clinically diagnosed by a well-defined set of patient symptoms, including jaundice, hepatic tenderness, and an increase in the serum levels of alanine aminotransferase and aspartate aminotransferase. Serologic immunoassays are generally performed to diagnose the specific type of viral causative agent. Historically, patients presenting with symptoms of hepatitis and not otherwise infected by hepatitis A, hepatitis B, Epstein-Barr or cytomegalovirus were clinically diagnosed as having non-A, non-B hepatitis (NANBH) by default.
For many years, the agent of non-A, non-B hepatitis remained elusive. It has now been established that many cases of NANBH are caused by a distinct virus termed hepatitis C virus (HCV). European Patent Application EP-A-0318216 discloses cDNA sequences derived from one strain of HCV, polynucleotide probes and polypeptides for use in immunoassays. Further information on that strain is provided in European Application EP-A-0388232.
The HCV genome encodes a large polyprotein precursor, which contains structural and non-structural regions. The single protein is apparently cleaved into a variety of proteins after production. Most of the structural and non-structural proteins have now been identified from in vitro RNA translation and expression as recombinant proteins. The C and E regions encode for nucleocapsid structural proteins and for envelope structural proteins, respectively. At least five additional regions follow, which encode for non-structural (NS) protein of undefined function. The organisation is believed to be as follows (A. Alberti, Journal of Hepatology, 1991; 12; 279 to 282)
5′3′NCR:C:E1:E2:NS1:NS2:NS3:NS4:NS5
Certain immunoreactive proteins have been described as recombinant proteins, for example C22 (in the core region), C33 (in NS3 region), 5-1-1 and C100 (both in the NS4 region), and NS5 (NS5 region). Current diagnosis of hepatitis C is often based on methods which detect antibodies against the product of the C-100 clone. This clone was produced by ligation of overlapping clones to produce a larger viral antigen (C100) corresponding to part of the NS3-NS4 genomic region. C100 was then fused with the human superoxide dismutase (SOD) gene, expressed in use as a large recombinant fusion protein (C100-3) and used on solid phase to develop radio-labelled (RIA) and enzyme-linked immunosorbent assays (ELISA).
Polynucleotides alleged to be useful for screening for HCV are disclosed in European Patent Specification EP-A-0398748. European Patent Specification EP-A-0414475 purports to disclose the propagation of HCV in culture cells and the production of antigens for use in diagnostics. European Patent Specification EP-A-0445423 discloses what is stated to be an improved immunoassay for detecting HCV antibodies.
Blood banks in the United Kingdom carry out routine testing of blood donors for antibodies to components of HCV. A first generation assay involved the detection of HCV antibodies to C100-3 polypeptides. The C100-3 antibody recognises a composite polyprotein antigen within non-structural regions of the virus and is a consistent marker of HCV infection. However, in acute infections this antibody is unreliable because of the delay (typically 22 weeks) in seroconversion after exposure. Furthermore, the C100-3 antibody test lacks specificity for the hepatitis C virus.
Second generation antibody tests employ recombinant antigens and/or synthetic linear peptides representing structural antigens from the highly conserved core region of the virus as well as non-structural antigens. However, it is found that some second-generation ELISA tests can yield false-positive reactions. The recombinant immunoblot assay (RIBA-2) incorporating four antigens from the HCV genome, purports to provide a method for identifying genuine-anti-HCV reactivity. However, the result can be “indeterminate”. The present workers have reported (The Lancet, 338; Oct. 19, 1991) varying reactivity of HCV-positive blood donors to 5-1-1, C100, C33 and C22 antigens, and compared these with the results of the direct detection of HCV RNA present in the blood samples using polymerase chain reaction (PCR) to amplify HCV polynucleotides. However, the work demonstrates that the unambiguous diagnosis of HCV infections is not yet possible.
Recently there has been discovered further types of HCV that differ considerably in sequence and these have been called HCV-2, 3 and 4. Our patent application WO93/10239 (published on 27th May, 1993) describes certain antigenic sequences of HCV-2, 3 and 4. The sequences disclosed for HCV-4 are in the 5′ NCR and core regions only. The former does not code for any protein which might be used in an immunoassay for HCV whilst the core region tends to be conserved.